A hydrocarbon drilling operation utilizes control and data collection equipment on the earth's surface and subsurface equipment such as a drilling assembly having drilling apparatus and formation evaluation tools that measure properties of the well being drilled. It has long been recognized in the oil and gas industry that communicating between the surface equipment and the subsurface drilling assembly is both desirable and necessary.
Downlink signaling, or communicating from the surface equipment to the drilling assembly, is typically performed to provide instructions in the form of commands to the drilling assembly. For example, in a directional drilling operation, downlink signals may instruct the drilling apparatus to alter the direction of the drill bit by a particular angle or to change the direction of the tool face. Uplink signaling, or communicating between the drilling assembly and the surface equipment, is typically performed to verify the downlink instructions and to communicate data measured downhole during drilling to provide valuable information to the drilling operator.
A common method of downlink signaling is through mud pulse telemetry. When drilling a well, fluid is pumped downhole such that a downhole receiver within the drilling assembly can meter the pressure and/or flowrate of that fluid. Mud pulse telemetry is a method of sending signals by creating a series of momentary pressure changes, or pulses, in the drilling fluid, which can be detected by a receiver. For downlink signaling, the pattern of pressure pulses, including the pulse duration, amplitude, and time between pulses, is detected by the downhole receiver and then interpreted as a particular instruction to the downhole assembly.
The concept of transmitting signals from the surface of the earth to subsurface equipment through mud pulse telemetry is known and has been practiced in the past. The most common method for creating pressure pulses is by interrupting drilling and cycling the drilling pump on and off at a certain frequency to create pressure pulses that travel downhole through the drill string to instruct the downhole assembly.
Another method combines pump cycling with rotation of the drill string. Drilling is interrupted, the drilling tool is lifted off bottom, and the pumps are cycled on and off to inform the downhole assembly that an instruction will be sent from the surface. Then the drill string is rotated at a given speed over a certain duration, and the downhole assembly includes a RPM sensor to measure the rotations. In this manner, instructions are communicated to the downhole assembly.
These transmission methods have several disadvantages. The most significant disadvantage is that drilling must be temporarily interrupted every time a signal is sent downhole. Thus, signals are sent downhole only periodically rather than continuously so that forward progress can be made in the drilling operation. During directional drilling, this can be particularly undesirable because the drilling tool can only be adjusted periodically resulting in an unwanted snake-like or tortuous borehole being drilled. Further, these methods are inherently slow because it takes time to start and stop the drilling operation, and although the goal is to instruct the downhole assembly by sending one set of signals, often the signals must be repeated since the downhole receiver does not always properly receive the instruction the first time. Finally, this method also causes unnecessary wear and tear to the pump and associated equipment.
Improved apparatus have been developed for transmitting command signals from the earth's surface to equipment downhole without starting and stopping the drilling system pumps. For example, U.S. Pat. No. 5,113,379 (“the '379 Patent”) to Scherbatskoy, hereby incorporated herein by reference for all purposes, describes creating negative pressure pulses by the sequential operation of a valve to bypass a quantity of the drilling fluid from the fluid being pumped downhole. The bypassed fluid is returned to the mud pit, and a surge absorber is employed to prevent backpressure in the mud return line from limiting the flow of fluid through the valve. This system has the disadvantage of not providing a means for adjusting the flowrate through the bypass line. Such flowrate adjustment is desirable for producing pulses of a particular amplitude and for ensuring that the bypass flowrate does not detract from the drilling fluid flowrate in such a way that the drilling operation is stalled.
The '379 Patent describes another method for creating pressure pulses by opening and closing a valve in communication with a reservoir having a different fluid pressure than the drilling system pump pressure. Again, this pulsing system provides no apparatus for controlling the flowrate through the pulsing system, and it has more complicated equipment requirements.
Still another method described in the '379 Patent requires a motor driven pump to be connected to the drilling system to introduce positive pressure pulses into the fluid column. Although this pulsing system allows for changes in flow rate based on the motor speed, the equipment requirements are more complicated, more expensive, and require more maintenance. Thus, it is desirable to provide a transmitter system for pulsing signals downhole that has simple, inexpensive, and easily maintainable equipment and that provides a way to adjust the flowrate of the bypass fluid.
European Patent Application EP 0 744 527 A1 (“the '527 Application”) filed by Baker-Hughes Incorporated, the contents of which are hereby incorporated herein for all purposes, discloses a simple bypass system for producing negative pressure pulses comprising a pneumatically actuated valve and an orifice. The orifice limits the flowrate through the bypass line, and the flowrate can further be adjusted by restricting flow through the valve itself. Further, the speed of the valve actuation is controllable for altering the frequency of the pulse signal.
Although the bypass system disclosed in the '527 Application provides an orifice for controlling the bypass flowrate, the orifice is not changeable to adjust the flow restriction as necessary. Namely, as a well is drilled deeper, a higher drilling flowrate is required to prevent the drilling tool from stalling. A change in flow resistance through the drill string may also be caused by, for example, bit jet changes, increased drill string length, and changes in the bottom hole assembly. Such flow resistance changes through the drill string require a change in the bypass flow resistance to maintain the desired bypass flowrate. Therefore, it is desirable to provide apparatus to adjust the bypass flowrate in the field. Restricting flow through the valve to adjust the bypass flowrate is not preferable because the valve internals will be eroded, and valves are costly to replace. Thus, it is desirable to include a low cost, sacrificial bypass flow restrictor that is easily changeable in the field to adjust the bypass flowrate.
Further, the invention disclosed in the '527 Application provides no component upstream of the bypass valve to reflect the positive pulses created each time the valve closes. This arrangement would pose problems if simultaneous, bi-directional communication (downlink and uplink) is desired because the positive pulses at the valve will travel upstream into the main piping and could interfere with or cancel out uplink pulses. Thus, it is desirable to provide pulse transmitter equipment arranged in such a way that simultaneous, bi-directional communication is achievable.
Once the pressure pulses representing a certain instruction are generated on the surface and transmitted downhole, a receiver disposed in the downhole assembly must decode those signals to distribute the instruction to the proper downhole tool. The receiver will detect noise associated with the pump and drilling operations in addition to the downlink signal. Therefore, decoding the downlink signal in the downhole receiver typically comprises digital filtering steps to remove the noise and using a detection algorithm to match the pressure pulse sequence to a particular pre-programmed instruction in the downhole assembly controller.
The '379 Patent describes in detail a method for analyzing uplink pulses. The data is first filtered and cross-correlated to remove pump pressure, pump noise, and random noise. Then the shape or duration of each pulse is analyzed to determine the data value associated with that pulse. With respect to downlink signals, the command signals are limited to a narrow frequency band over a particular time interval. Therefore, the relevant quantity for the receiving system is the frequency band and time of reception for the received signal. The signal passes through a lock-in amplifier filter to separate the narrow-band frequency signal from interfering noise. Then the signal passes to an amplifier and to a pulse generator, which feeds the coil of a stepping switch, preferably electronic, to step the switch for various instrument functions.
These uplink and downlink telemetry systems employ filters and algorithms for analyzing the signals, but the uplink system is significantly more sophisticated. Uplink transmission is said to involve large amounts of data that must be analyzed quickly, whereas downlink transmission is said to involve small amounts of data that can be analyzed over a longer time frame. For example, the stated data rate for uplink signals is about 120 bits per minute whereas the stated data rate for downlink signals is up to 1 bit per minute, thus requiring less power for transmission. Further, the noise downhole is said to be lower than the noise near the surface, so the filtering feature is not as complicated downhole.
However, given the complicated functionality of modem day drilling assemblies, and especially in directional drilling applications, it is desirable to have fast data rates for both uplink and downlink communications. Further, it is desirable to provide a sophisticated downlink algorithm capable of fast and accurate signal decoding, including an internal error-checking capability. In fact, it is desirable to achieve simultaneous, bi-directional communication (uplink and downlink) to send a downlink instruction that is decoded quickly, confirmed via uplink, and executed in fast progression, such that while one downlink instruction is being executed another downlink signal can be sent—either to the same tool or to a different tool. In directional drilling applications, the benefit of a fast bi-directional telemetry rate is the drilling of a very accurately located borehole that may be optimized for minimum drag since the drill bit angle and tool face can be corrected rapidly whenever it goes off course. The downlink telemetry system of the present invention overcomes the deficiencies of the prior art.